This was mostly just me pulling a bunch of cards from decks I run though so obviously I dont know what sort of stiutations you are in or what would work specifically for you.
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It really depends what kind of utility you want. There's a lot of cheap creature/artifact kill and recursion in these colors, but not a ton of card draw or mana ramp, which is what decks often want. You've got Sisters of Flame, Iron Myr, and Lead Myr if you're desperate for creature-based ramp. Moriok Replica and Phyrexian Rager draw cards. Dark Confidant is surprisingly good in EDH, especially if you can bring your curve down.
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Guildmages are sweet, I'm a big fan of 2/2 beats in all formats. Plaguebearer is a nice elf/token killer. Undertaker is solid recursion. Academy Raider is the red Looter il-Kor. Viashino Heretic and Withered Wretch have been mentioned but they really are great. Jaya Ballard is great, even if there isn't much blue in your meta. Squee, Goblin Nabob synergizes well with looting effects and spellshapers, and he chumps forever. Taureen Mauler doesn't do much but get huge, but he does that well. Goblin Diplomats speeds the game up real nice. Doomed Necromancer is an ok reanimation trick. Skirsdag High Priest will do a ton of work with all these extra cheap guys.
I know the feeling TC. It's more fun to look at your Titans, and Dragons, and Eldrazis (oh my!), especially with a commander like Rakdos, Lord of Riots (assuming this is the one you meant, but if it's The Defiler then my apologies).
The key is to pick out the, say, six or seven huge drops you want since these are uncastable until the end game - in Rakdos I'd stick with It That Betrays, Ulamog/Kozilek, Grave Titan and a Wurmcoil Engine. Take all your other big, dumb dudes and set them aside for another project (or take turns alternating them into the line-up). Consider then small cheap burn as removal that also turns on your commander or finishes opponents - and with this burn take a look at Satyr Firedancer. Additionally, take a look at 1v1 lists for Lyzolda, the Blood Witch - I know you're into multiplayer, but there are some amazing suggestions in those threads, in addition to the suggestions others here have already given you.
It doesn't look like anyone has mentioned Abyssal Gatekeeper yet; it's a pretty solid rattlesnake, and gets more useful if you have any sacrifice effects.
Dark Confidant is about as good as it gets. He's better in the format than people give him credit for, most decks have a much lower actual curve than they think once you take lands into account.
When in doubt, ramp it out. There's not a lot of ramp in your colors, so you will have to rely heavily on mana rocks. I just started deckbuilding in Theros standard, so forgive me if this list is a bit limited (or if the suggestions seem subpar, I'm a n00b at EDH, too):
Chromatic Lantern (my favorite, as it also fixes you in your colors by allowing your lands to produce mana of your commander's colors) Rakdos Keyrune Rakdos Cluestone Darksteel Ingot Crypt Ghast (downside = CMC-4, but doubles your Swamps early enough to give you a boost) Liliana of the Dark Realms (she's CMC-4, but she fetches Swamps, and her ultimate is silly--plus she has the added benefit of being able to nuke creatures) Astral Cornucopia (jury is still out on whether this is good or not, meaning it's in all my decks but I've never actually seen it yet) Sol Ring
Black also has things like Dark Ritual. Red has that temporary boost, too, in things like Akki Rockspeaker. I would look on the Gatherer to find more options; that is what I'm doing right meow to find you suggestions!
I'm not quite sure why, but I'm very particular about my ramp. I like to be sure I have 8-10 cards per deck that are specifically for ramp (no, I do not always play green in my decks). The other members of my play group have learned long ago to leave me alone about it; they can't complain when I win 50% of the matches because of my ability to start dropping six-mana bombs on turns four and five.
If you're not interested (or lack the card base for) mana ramp, how about some Rakdos control? Burn spells (Lightning Bolt), kill spells (Terror, Hero's Downfall), discard spells (Thoughtseize) are decent options for 1v1 or some spot removal for multi-player games. Damnation may be an option, too, depending on what you're doing with your deck.
1 Goblin Matron
1 Hellraiser Goblin
1 Pilgrim's Eye
1 Burnished Hart
1 Coffin Queen
1 Fleshbag Marauder
1 Dark Impostor
1 Stinkweed Imp
1 Crypt Rats
This was mostly just me pulling a bunch of cards from decks I run though so obviously I dont know what sort of stiutations you are in or what would work specifically for you.
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goblin welder like ISBPathfinder suggest. (gets outta control quite fast)
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for 1v1: Hypnotic Specter
for multiplayer: Liliana's Specter
Viashino Heretic
It really depends what kind of utility you want. There's a lot of cheap creature/artifact kill and recursion in these colors, but not a ton of card draw or mana ramp, which is what decks often want. You've got Sisters of Flame, Iron Myr, and Lead Myr if you're desperate for creature-based ramp. Moriok Replica and Phyrexian Rager draw cards. Dark Confidant is surprisingly good in EDH, especially if you can bring your curve down.
Currently Playing:
Legacy: Something U/W Controlish
EDH Cube
Hypercube! A New EDH Deck Every Week(ish)!
The key is to pick out the, say, six or seven huge drops you want since these are uncastable until the end game - in Rakdos I'd stick with It That Betrays, Ulamog/Kozilek, Grave Titan and a Wurmcoil Engine. Take all your other big, dumb dudes and set them aside for another project (or take turns alternating them into the line-up). Consider then small cheap burn as removal that also turns on your commander or finishes opponents - and with this burn take a look at Satyr Firedancer. Additionally, take a look at 1v1 lists for Lyzolda, the Blood Witch - I know you're into multiplayer, but there are some amazing suggestions in those threads, in addition to the suggestions others here have already given you.
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Chromatic Lantern (my favorite, as it also fixes you in your colors by allowing your lands to produce mana of your commander's colors)
Rakdos Keyrune
Rakdos Cluestone
Darksteel Ingot
Crypt Ghast (downside = CMC-4, but doubles your Swamps early enough to give you a boost)
Liliana of the Dark Realms (she's CMC-4, but she fetches Swamps, and her ultimate is silly--plus she has the added benefit of being able to nuke creatures)
Astral Cornucopia (jury is still out on whether this is good or not, meaning it's in all my decks but I've never actually seen it yet)
Sol Ring
Black also has things like Dark Ritual. Red has that temporary boost, too, in things like Akki Rockspeaker. I would look on the Gatherer to find more options; that is what I'm doing right meow to find you suggestions!
I'm not quite sure why, but I'm very particular about my ramp. I like to be sure I have 8-10 cards per deck that are specifically for ramp (no, I do not always play green in my decks). The other members of my play group have learned long ago to leave me alone about it; they can't complain when I win 50% of the matches because of my ability to start dropping six-mana bombs on turns four and five.
If you're not interested (or lack the card base for) mana ramp, how about some Rakdos control? Burn spells (Lightning Bolt), kill spells (Terror, Hero's Downfall), discard spells (Thoughtseize) are decent options for 1v1 or some spot removal for multi-player games. Damnation may be an option, too, depending on what you're doing with your deck.
Looking specifically for creatures, most of the good ones were mentioned, but might throw Goblin matron on there situationally.
UW Ephara Hatebears [Primer], GB Gitrog Lands, BRU Inalla Combo-Control, URG Maelstrom Wanderer Landfall